Current source for supplying a current having an exponential wave form

ABSTRACT

By using an operational amplifier, a few resistors and a reactance a current source is obtained which supplies a current having an exponential waveform. The current source may be used in analogue computers for operations with logarithms, in apparatus for testing semiconductors and in measuring instruments provided with a logarithmic display.

United States Patent 1 1 3,753,131

Kramer 1 1 Aug. 14, 1973 CURRENT SOURCE FOR SUPPLYING A 3,521,082 7 1970Woik 328/145 CURRENT HAVING AN EXPQNENTIAL 3,113,274 12/1963 Utt 328/144WAVE FORM 3,436,673 4/1969 Fricke 330/104 [75] inventor: Pieter Kramer,Nijmegen, FQREIGN PATENTS OR APPLICATIONS Netherlands 910,427 7/1949Germany 330/104 [73] Assignee: U.S. Philips Corporation, New OTHERPUBLICATIONS York, NY, Levin, Band Switching an Active Filter page 611,IBM Tech. Disclosure Bulletin Vol. l3, Aug. 1970. F l d: 71 [22] Dec 3019 PhilbricklNexus Research Applications Manual for [21] Appl. No.:214,286 OP Amps 1968, pages 53, 54, 66, 86, 96, 97, and 102.

[30] Foreign Application Priority Data Z' g i Hucken ssistant xaminerartJan. 8, 1971 Netherlands 7100277 Attorney-Frank R. Trifari [52] U.S. Cl.328/145, 330/69 [51 1 int. Cl G06g 7/24 [57] ABSTRACT 0' Search 142,143, y i g an operational amplifier a few resistors and 328/144 167;330/69- l R a reactance a current source is obtaineci which supplies acurrent having an exponential waveform. The current [56] ReferencesCited source may be used in analogue computers for opera- UNITED STATESPATENTS tions with logarithms, in apparatus for testing semicon-3,696,304 10/1972 Fricke 330/1 A uc ors and in measuring instrumentsprovided with a 3,477,034 11/1969 Gioia 330/69 logarithmic display.3,237,002 2/1966 Patmore 330/40 3 Claims, 3 Drawing Figures CURRENTSOURCE FOR SUPPLYING A CURRENT HAVING AN EXPONENTIAL WAVE FORM Theinvention relates to a current source for producing a current having anexponential. wave form, which circuit comprises a first and a secondterminal for connection to a load, a reactance, resistors and adifferential amplifier provided with a first and. a second, inputterminal, an output terminal and a common lead.

Current sources are known. They generally are in the form of atwo-terminal device between which a load may be connected so that theload passes a current which within certain limitsis independent of thevalue of the load. Usually the current source comprises a high-tensionvoltage supply having a large internal resistance, so that in the caseof comparatively small loads the current is determined by the voltageand the high internal resistance.

For some purpose it may be necessary for the current delivered by acurrent source to satisfy certain conditions, for example to have acertain time dependence or to be dependent on a given parameter. in thiscase a function generator may be used which produces the requiredfunction and delivers it output quantity, usually a voltage, to a finalstage which is connected as a current source.

it is known to produce exponential or logarithmic time functions bymeans of charging or discharging capacitors, generally in conjunctionwith a fixed voltage and a series resistor. Inductors may also be usedin conjunction with voltage supply source and series resistors.

It is an object of the invention to provide a very simple circuit whichhas a current supply output at which a current is supplied which veriesexponentially with time, without using succeeding function generatorsand current source cirucits.

For this purpose a current source for supplying a current having anexponential waveform of the type described at the beginning of thespecification is characterized in that the output terminal is connectedthrough a first resistor to the first input terminal and through thereactance to the second input terminal, the first load connectionterminal being connected through a second resistor to the first inputterminal and through a third resistor to the second input terminal, thecommon lead being connected to the second load connection terminal.

The differential amplifier may be of the operational amplifier type,providing large amplification and a high input impedance.

Depending upon the voltage drive of the amplifier and the initialcondition, the current source circuit according to the invention enablesan exponential waveform extending over several decades to be obtainedwith a satisfactory degree of accuracy. if a capacitor is used as thereactance, the initial condition may simply be set by imparting aninitial charge to the capacitor via switches. in the formula I Ac, whereI is the current of the circuit and e is the base of the naturallogarithm, the constant A is determined by the said initial charge andby the values of the capacitor and the resistors, whilst B is inverselyproportional to a time constant fixed by these values. The capacitor maybe replaced by an inductor to which an initial current is supplied tofix the initial condition.

Embodiments of the invention will now be described, by way of example,with reference to the accompanying diagrammatic drawings, in which:

FIG. 1 is a circuit diagram of the current source according to theinvention,

FIG. 2 is a circuit diagram of a current source using a capacitor and astarting circuit, and

FIG. 3 is a circuit diagram of an embodiment using an inductor and anassociated starting cirucit.

Referring now to FIG. 1, the current source shown has terminals 1 and 2load B is to be connected. A differential amplifier 3. having inputterminals 4 and 5 and a common lead 6 is fed from supply sources, notshown. An output terminal 7 supplies current which through a reactance Zand a resistor R and through the series combination of resistors R andR, is available to the load B at the terminal 1. The current returns viathe terminal 2 and the lead 6. The reactance Z is connected to the inputterminal 4 and to the output terminal 7 of the amplifier, wilst theresistor R is connected between the input terminal 4 and the terminal 1.The inputterminal 5 is connected to the junction point of the resistorsR, and R,.

in FlG. 2 elements corresponding to those of the circuit of FIG. 1 aredesignated by like reference numerals. The reactance here is a capacitorC which is shunted by a switch 8, which in the operative positionshort-circuits the capacitor and hence sets the current source circuitto the rest condition in which the terminals l and 2 carry no current.

The switch 8 is coupled to a switch 9 capable of switching a capacitor10. In the rest position of the two switches, which position is shown inthe Figure, the capacitor 10 is connected in parallel with the capacitorC. In the operative positions of the two switches, which aremechanically coupled to one another, the capacitor 10 is charged from adirect-voltage supply source connected to terminals 11 and 12. The valueof the capacitor i0 is very small compared with that of the capacitor C,so that when determining the time constant of the circuit the lattercapacitor only need be taken into account and on the other hand when theswitches 8 and 9 are in their rest positions the charge distributionbetween the two capacitors is such that when the circuit is started onlya small voltage is set up across the capacitor C, for example a voltageof the order of microvolts or at most millivolts, whilst at the sametime the directvoltage supply source voltage across the terminals 11 and12 may be, say, 12 volts.

A calculation of the current I shows that, neglecting the influence ofthe amplification factor and the input resistance of the amplifier whichare assumed to be very large, the exponential function is: l m- 2 s/ is) r a (I) where I is the current, U the initial voltage across thecapacitor C at starting, R,, R,, R, and C are the values of the elementsdesignated by these references, and t is time, the time constant in the2 power being given by The initial voltage U, may alternatively bedirectly applied to the capacitor C via change-over switches by aseparate supply source.

in the circuit shown in FIG. 3 the reactance is a coil L having aninductance L. In this case the current satisfies the relationship: I:(R2 a z) e l 3 2 where i,,, is the initial current through the coilwhich is necessary when starting the circuit. The time constant is givenby:

By means of a switch 8 the coil is short-circuited and the circuit isbrought to the starting condition in which the current I is zero. Inthis condition the current i,,, is produced in the magnetic circuit inthe coil by a current in a separate winding 13 which is supplied via aswitch 9 and a resistor 14 by a source connected to the terminals 1 land 12. Since the switches 8 and 9 are mechanically coupled to oneanother, at the starting of the circuit the short-circuit of the coil Lis removed and a current i,,, is produced in the coil L.

The zero point of the current I at the instant r=0 may be shifted bysupplying an additional current from a separate current supply to theterminals 1 and 2. Thus, there may be supplied to a load a currenthaving the nature I =const. (l e').

The current supply circuit according to the invention may be used toadvantage in analogue computer circuits, an apparatus for testingsemiconductors, where an e-power function is frequently met with, and inmeasuring instruments in which a logarithmic display of measurements isdesired.

What is claimed is:

1. Current source for producing a current having an exponentialwaveform, which circuit comprises a first and a second terminal forconnection to a load, a reactance, resistors and a differentialamplifier provided with a first and a second input terminal, an outputterminal and a common lead, characterized in that the output terminal isconnected through a first resistor to the first input terminal andthrough the reactance to the second input terminal, the first loadconnection terminal being connected through a second resistor to thefirst input terminal and through a third resistor to the second inputterminal, whilst the common lead is connected to the second loadconnection terminal.

2. Current source as claimed in claim 1, characterized in that thereactance is a capacitor the capacitance value of which together withthe first, second and third resistors determines the time constant ofthe exponential waveform and which by means of switches may beshort-circuited to return the current source to the starting positionand also may be provided with an initial charge by means of which, whenthe current is started, and by means of the values of the capacitor andthe resistor the proportionality constant of the current may be fixed.

3. Current source as claimed in claim 1, characterized in that thereactance is an inductor the value of which together with the first,second and third 9 resistors determines the time constant of theexponential waveform and which by means of switches may beshort-circuited to return the current source to the starting positionand also may be provided with an initial current i so that, when thecurrent is started, by this initial current and by the values of theinductor and the resistors the proportionality constant of the circuitcurrent may be fixed.

# t i i UNiTED STATESPATENT OFFICE ls/6e) 1 I,

CERTEMQATE 0F CORRECTION Patent l I Dated August 14, 1973 PIETER KRAMERInventofls) a p It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

F'connecting the amplifier output terminal through a first "1 resistorto the first input terminal and through the reactance to the secondinput terminal, means connecting the first load connection terminalthrough a second resistor to the first input terminal and through athird resistor to the second input terminal, and means connecting thecommon lead to the second load connection terminal.

2, current source as claimed in claim'il, character ized in that thereactance comprises a capacitor having a capacitance value which,together with the first, second and third resistors determines the timeconstant of the exponential waveform, and switching means by means ofwhich the capacitor may be short-circuited to return the current sourceto a starting position and by which the capacitor also may be providedwith an initial charge so that, when the current is started, and

Livy n'aeans of the values of the capacitor and the resistors thertionality constant of the current may be fixed.

were STATES FATENT emcr (5/69) l q I CERTIFEEATE (9F QQRREUFIQN PatentNo. 3,753,131 hated Auqust 14,1973

InVnt -N PIETER KRAMER It is certified that error appears in e'boveidentified patent and that said Letters Patent are hereby corrected asshown below:

col. 1, line 5, cancel which 7 .1

line 6, cancel "circuit" and insert and more particularly to a currentsource which line 7, before "resistors" insert some line 19, change"purpose" to purposes line. 21, after "example" insert a comma 7 line24'," change it to its line '32, change "source" to sources line 35,"change "veries" to varies line 64," cancel "whilst" and insert whilecol. 2, line 11, after "2 insert to which a line iii/ cancel fed" andinsert energized line 20, cancel wilst" and insert and line 41, "cancelthe comma h) 7 after that" insert a. comma L line 42fbefore the" inserta comma 7 J I line 43 cancel "and on and insert n -7 72 3 UNITED sm'rrssMYENT OFFIQE CERTIFICATE OF QORRECTION Patent No. 3,753, 131 DatedAugust 14, n 1973 Invehtofls) PIETER KRAMER It is certified that errorappears in the above-identified patent and that said Letters Patent arehereby corrected as shown below:

col. 2, line 44, after "positions" insert a comma 1 line 47 before "a"insert a comma line 48, cancel whilst at" and insert At line 55, cancel"e (R /R R C) t" and insert (R /R R C) t col. 3, line 4, cancel "'3'. iand insert I 11 cancel "e (R R /R L) t" and insert e (R R /R n) t line30, cancel "an" and insert in IN THE CLAIMS Claims 1-3 should read asfollows:

1. A current source for producing a current having an exponentialwaveform comprising, a first and a second terminal for connection to aload a reactance a luralit of resistors a differential amplifierprovided with a first and a second input terminal, an output terminaland a common lead, means .1

73x33 1 Hearts stuns PATENT smut CERTiFiCATE HF EQRRECTIUN Patent 2 75313:1 k Dated Auqust 14. 1973 Inventofls) PIETER KRAMER i It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as show: below:

3'; A current source as claimed in claim 1,

characterized in that the reactance comprises an inductor having aninductance value which, together with the first,

second and third resistors determines the time constant of theexponential waveform, and switching means for short-circuiting theinductor to return the current source to a starting position and alsofor providing the inductor with an initial current i so that, when thecurrent is started, the initial current and the values of the inductorand the resistors fix the proportionalityconstant of the current.

Signed and seaied this 1st day'of Getober 1974.

{SEAL} attest:

MeCOY Mo GIBSON JRo 6., MARSELQLL DANN Attesting Officer Commissioner ofPatents

1. Current source for producing a current having an exponentialwaveform, which circuit comprises a first and a second terminal forconnection to a load, a reactance, resistors and a differentialamplifier provided with a first and a second input terminal, an outputterminal and a common lead, characterized in that the output terminal isconnected through a first resistor to the first input terminal andthrough the reactance to the second input terminal, the first loadconnection terminal being connected through a second resistor to thefirst input terminal and through a third resistor to the second inputterminal, whilst the common lead is connected to the second loadconnection terminal.
 2. Current source as claimed in claim 1,characterized in that the reactance is a capacitor the capacitance valueof which together with the first, second and third resistors determinesthe time constant of the exponential waveform and which by means ofswitches may be short-circuited to return the current source to thestarting position and also may be provided with an initial charge bymeans of which, when the current is started, and by means of the valuesof the capacitor and the resistor the proportionality constant of thecurrent may be fixed.
 3. Current source as claimed in claim 1,characterized in that the reactance is an inductor the value of whichtogether with the first, second and third 9 resistors determines thetime constant of the exponential waveform and which by means of switchesmay be short-circuited to return the current source to the startingposition and also may be provided with an initial current i0 so that,when the current is started, by this initial current and by the valuesof the inductor and the resistors the proportionality constant of thecircuit current may be fixed.